"Long before it's in the papers"
January 27, 2015


Past global warmings were good times for sea crocs

Aug. 19, 2014
Courtesy of the University of Bristol 
and World Science staff

Past episodes of nat­u­rally caused glob­al warm­ing were gold­en op­por­tun­i­ties for sea croc­o­diles to spread, ac­cord­ing to a new stu­dy. The work found that an­ces­tors of to­day’s croc­o­diles col­o­nized the seas dur­ing warm phases and be­came ex­tinct dur­ing cold ones.

Artist's il­lus­tra­tion of a type of ma­rine croc­o­dil­ian called dy­ro­sau­rid, near the end of the Cre­ta­ceous pe­ri­od. (Cred­it: Guil­laume Suan)

To­day, croc­o­diles are so-called cold-blood­ed an­i­mals liv­ing mainly in fresh wa­ters, though two spe­cies are ex­cep­tions that ven­ture oc­ca­sion­ally in­to the sea—Cro­co­dy­lus po­ro­sus and Cro­co­dy­lus acu­tus. Be­cause croc­o­diles live in trop­i­cal cli­mates, their fos­sils are of­ten used as mark­ers of warm con­di­tions.

While only 23 croc­o­dile spe­cies live to­day, there were hun­dreds in the past, ac­cord­ing to sci­en­tists. And four times in the past 200 mil­lion years, ma­jor lin­eages en­tered the seas, then died out.

“There seems to be a com­mon pat­tern” be­hind these events, said Jer­e­my Mar­tin, lead au­thor of the new stu­dy, pub­lished this week in the jour­nal Na­ture Com­mu­nica­t­ions. The work “shows that for cro­c­o­dil­ians, warm­ing phases of our earth’s his­to­ry con­sti­tute ide­al op­por­tun­i­ties to col­o­nize new en­vi­ron­ments,” added co-au­thor Mi­chael Ben­ton of the Uni­vers­ity of Bris­tol.

Mar­tin, from the Uni­vers­ity of Ly­on in France and form­erly of the Uni­vers­ity of Bris­tol, U.K., worked with pa­le­on­tol­ogists and geo­chemists from both in­sti­tu­tions to chart the num­ber of ma­rine croc­o­dil­ian fos­sil spe­cies as com­pared to sea tem­per­a­tures. To map these past tem­per­a­tures, the sci­en­tists ex­ploited past find­ings show­ing that the tem­per­a­tures af­fect the atom­ic struc­ture of the ox­y­gen in fish fos­sils.

They found that croc­o­diles first col­o­nized the seas about 180 mil­lion years ago dur­ing a pe­ri­od of ocean glob­al warm­ing. These an­i­mals died about 25 mil­lion years lat­er dur­ing a cold spell, but an­oth­er croc­o­dil­ian line­age re-col­o­nized the ma­rine en­vi­ron­ment dur­ing a lat­er pe­ri­od of warm­ing. But one fos­sil line­age does­n’t seem to fol­low the trend, they said—Juras­sic metri­orhyn­choids did­n’t die out dur­ing the cold spells of the early Cre­ta­ceous, un­like the teleo­saurids, an­oth­er group of ma­rine crocodil­ians.

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Past episodes of naturally caused global warming have served as golden opportunities for sea crocodiles to spread, according to a new study. The research found that ancestors of today’s crocodiles colonized the seas during warm phases and became extinct during cold ones. Today, crocodiles are so-called cold-blooded animals living mainly in fresh waters, though two species are exceptions that venture occasionally into the sea—Crocodylus porosus and Crocodylus acutus. Because crocodiles live in tropical climates, their fossils are often used as markers of warm conditions. While only 23 crocodile species live today, there were hundreds in the past, according to scientists, and four times in the past 200 million years major lineages entered the seas, then died out. “There seems to be a common pattern” behind these events, said Jeremy Martin, lead author of the new study, published this week in the journal Nature Communications. The work “shows that for crocodilians, warming phases of our earth’s history constitute ideal opportunities to colonize new environments,” added co-author Michael Benton of the University of Bristol. Martin, from the University of Lyon in France and formerly of the University of Bristol, U.K., worked with paleontologists and geochemists from both institutions to chart the number of marine crocodilian fossil species as compared to sea temperatures. To map these past temperatures, the scientists exploited past findings showing that the temperatures affect the atomic structure of the oxygen in fish fossils. They found that crocodiles first colonized the seas about 180 million years ago during a period of ocean global warming. These animals died about 25 million years later during a cold period, but another crocodilian lineage re-colonized the marine environment during a later period of warming. One fossil lineage doesn’t seem to follow the trend, though—Jurassic metriorhynchoids didn’t go extinct during the cold spells of the early Cretaceous, unlike the teleosaurids, another group of marine crocodilians.